A Multi-Tracer Approach for Determining Sources of Nitrate
Contamination of Ground Water and Springs, Lafayette County, Florida

Brian G. Katz1,Johnkarl F. Bohlke2,andH. David Hornsby3

Wastes from animal farming operations (milk and beef cows, poultry, and
swine) can potentially contribute large quantities of nitrogen (N) to ground
water in Lafayette County, a rural area in northern Florida and one of the
leading producers of milk and broiler chickens in Florida. During 1955-95, N
inputs estimated from animal wastes (not corrected for losses due to
volatilization and waste-handling practices) accounted for 28 to 53 percent of
the estimated total N inputs (1.4 to 4.6 million kilograms per year) from all
sources of N (fertilizers; atmospheric deposition; wastes from cows, poultry,
and swine; and septic tanks). A multi-tracer approach, which consisted of the
analysis of spring-water and shallow ground-water samples for naturally
occurring chemical and isotopic tracers (

d15N, d18O, dD, d13C, CFCs, tritium) was used to determine sources and chronology of
nitrate contamination of ground water in Lafayette County and other parts of the
Suwannee River Basin. Water samples from six springs in Lafayette County [flows
greater than 280 liters per second (L/s)] had d15N-NO3
values ranging from 5.4 to 9.1 per mil, likely indicating a mixture of inorganic
and organic sources of nitrogen. Nitrate-N concentrations in spring waters
ranged from 1.7 to 5.5 milligrams per liter (mg/L). Springs integrate ground
water from large parts of the aquifer and mixing of waters from various
convergent flow paths is reflected by the separation in apparent ages determined
from measured concentrations of CFC-11 and CFC-113. Estimated residence times
for ground water discharging to springs range from 15 to 77 years, based on CFC
concentrations and the use of different flow-system models. Increases in nitrate
concentration in water samples from Troy Spring (flow greater than 2,800 L/s)
during 1960-98 track the increase in estimated fertilizer N inputs through the
early 1980's followed by the increase in estimated N inputs from animal wastes
during the mid-1980's to 1998.

In contrast, water from wells in the Upper Floridan aquifer (sampled zones
were 7-13 meters (m) and 26-32 m depth below land surface) had

d15N-NO3
values of 10.2 to 12.8 per mil, indicating an organic source of N. Ground-water
ages ranged from 8-16 years based on measured CFC-113 concentrations and a
piston-flow model. Nitrate-N concentrations in ground water were 18-20 mg/L
during low-flow conditions (July 1997) in the Suwannee River, but decreased to
10-13 mg/L after a period of prolonged rainfall (March 1998). Slightly elevated
concentrations of N2 gas indicate that denitrification reactions may
account for some of the decrease in NO3 concentrations during
high-flow conditions. Future studies in this area would benefit from the
analysis of animal pharmaceuticals and their metabolites in ground water in an
attempt to discriminate among various animal-waste sources of nitrogen, which
cannot be done using nitrogen-isotope data alone.